The present disclosure relates to ohmic fluid heating devices, and methods of heating a fluid. An ohmic fluid heater can be used to heat an electrically conductive fluid as, for example, potable water. Such a heater typically includes plural electrodes spaced apart from one another. The electrodes are contacted with the fluid to be heated so that the fluid fills the spaces between neighboring electrodes. Two or more of the electrodes are connected to a power supply so that different electrical potentials are applied to different ones of the electrodes. For example, where an ohmic heater is operated using normal AC utility power such as that obtainable from a household electric plug, at least one of the electrodes is connected to one pole carrying an alternating potential, whereas at least one other electrode is connected to the opposite pole. Electricity passes between the electrodes through the fluid at least one space between the electrodes, and electrical energy is converted to heat by the electrical resistance of the fluid.
It is desirable to control the rate at which electrical energy is converted to heat, (the “heating rate”), in such a heater to achieve the desired temperature of the heated fluid. It has been proposed to vary the heating rate by mechanically moving electrodes closer relative to one another, thereby varying the electrical resistance between the electrodes. Such arrangements, however, require complex mechanical elements including moving parts exposed to the fluid. Moreover, it is difficult to make such mechanisms respond quickly to deal with rapidly changing conditions. For example, if an ohmic heater is used in an “instantaneous heating” arrangement to heat water supplied to a plumbing fixture such as a shower head, the water continually passes through the heater directly to the fixture while the fixture is in use. If the user suddenly increases the flow rate of the water, as by opening a valve on the fixture, the heater should react rapidly to increase the heating rate so as to maintain the water supplied to the fixture at a substantially constant temperature.
It has also been proposed to provide an ohmic heater with a substantial number of electrodes and with power switches to selectively connect different ones of the electrodes to the poles of the power supply. For example, an array of electrodes may be disposed in a linear arrangement with spaces between the electrodes. The array includes two electrodes at the extremes of the array and numerous intermediate electrodes between the two extreme electrodes. To provide a minimum heating rate, the extreme electrodes are connected to opposite poles of the power supply, and the intermediate electrodes are isolated from the poles. The electric current passes from one extreme electrode through the fluid in a first space to the nearest one of the intermediate electrodes, then through fluid in the next space to the next isolated electrode and so on until it reaches the last intermediate electrode, and flows from the last intermediate electrode to the other extreme electrode. Thus, the fluid within all of the spaces is electrically connected in series between the two extreme electrodes. This connection scheme provides high electrical resistance between the poles of the power supply and a low heating rate.
For a maximum heating rate, all of the electrodes are connected to the poles so that each electrode is connected to the opposite pole from its next nearest neighbor. In this condition, the fluid in each space is directly connected between the poles of the power supply, in parallel with the fluid in every other space. The connection scheme provides minimum resistance between the poles. Intermediate heating rates may be achieved by connecting various combinations of electrodes to the poles of the power supply. For example, in one such connection scheme, two of the intermediate electrodes are connected to opposite poles of the power supply, and the remaining electrodes are electrically isolated from the poles of the power supply. The connected intermediate electrodes are separated from one another by a few other intermediate electrodes and a few spaces, so that fluid in only a few spaces is connected in series between the poles. This connection scheme provides a resistance between the poles that is higher than the resistance in the maximum heating rate scheme, but lower resistance than the resistance in the minimum heating rate scheme. With fluid having a given conductivity, different connection schemes will provide different resistances between the poles, and thus different heating rates.
Typically, the switches are electrically controllable switches such as semiconductor switching elements as, for example, thyristors. Ohmic heaters of this type can switch rapidly between connection schemes and thus switch rapidly between heating rates. Such heaters do not require any moving parts in contact with the fluid to control the heating rate. Ohmic heaters of this type can only select from among the set of the specific resistances fixed by the physical configuration of the electrodes, and thus the heating rate, in steps. As disclosed in U.S. Pat. Nos. 7,817,906 and 8,861,943, the disclosures of which are hereby incorporated by reference herein, the electrodes of such a heater can be spaced at non-uniform distances from one another to provide a wide range of resistances with fluid of a given conductivity and a large number of steps with substantially uniform ratios of heating rate between steps. As disclosed in International Application PCT/US2017/060192, the disclosure of which is hereby incorporated by reference herein, even more steps of heating rate can be provided with a given number of electrodes by providing shunting switches which can selectively connect certain ones of the electrodes to one another. In heaters of this type, the available switch combinations and the associated heating rates may be stored in a lookup table. Heaters of this type typically have been controlled by a feedback control system which reacts to operating conditions by selecting a higher or lower heating rate. For example, such a heater may include an outlet temperature sensor. If the fluid discharged from the heater is at a temperature below the desired temperature (also referred to as the “set point” temperature), the control system selects a combination of electrodes with a higher heating rate. Heaters of this type can provided effective heating, and can compensate for differences in operating conditions such as differences in flow rate, conductivity, inlet temperature.
However, still further improvement would be desirable.